#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

int main()
{
    int score = 0;
    //char grade='0';
    
    scanf("%d", &score);
    int x = score / 10;
    if (score == 100 || x == 9)
        printf("A\n");
    else if (x == 8)
        printf("B\n");
    else if (x == 7)
        printf("C\n");
    else if (x == 6)
        printf("D\n");
    else if (score >= 0 && score < 60)
        printf("E\n");
    else
        printf("F\n");
    return 0;
}